The present invention relates to a process for manufacturing a high quality prepreg having high thickness precision, exhibiting excellent formability, and suffering from only minimal fall out of resin powder. Particularly, a prepreg obtained by a process of the present invention can be suitably used for preparing substrates for cellular phones, personal computers, Rambus memories, PDAs, and the like for which high-frequency characteristics are required.
Recently, as higher frequencies than before are used for laminated sheets for circuit boards and multiple-layer circuit boards, the requirements for the properties of the materials used for such boards have become more stringent. Materials for circuit boards have conventionally been studied, particularly with respect to circuit signal delays. Resins capable of controlling impedance through a reduction in the dielectric constant of circuit boards and the increase in the thickness precision of the substrates after fabrication have been developed. The thickness precision of a substrate correlates with signal delays, and the square root of the dielectric constant correlates with the signal delays. Therefore, increasing the thickness precision of a substrate is an important subject. It has been difficult to provide conventional prepregs with high substrate thickness precision due to flow out of the resins impregnated and cured in glass fiber during press fabrication. Only poor substrate thickness precision can be achieved if the newest type press facility is not used. In addition, resin powder readily falls from prepregs when the prepregs are cut or bent during handling. Such a resin powder adheres to copper foils and causes circuit defects.
Methods for improving the substrate thickness precision have been disclosed in Japanese Patent Applications Laid-open No. 123875/1978, No. 142576/1979, No. 168438/1988, and No. 119836/1992. Japanese Patent Applications Laid-open No. 123875/1978, No. 142576/1979, and No. 119836/1992 disclose prepregs containing a completely cured resin layer and a partly cured resin layer. The dimensional stability of laminated boards has been improved by using these methods. However, these methods have a problem of peel-off of the resin layers at the interface of a completely cured resin layer and a partly cured resin layer. Japanese Patent Application Laid-open No. 168438/1988 discloses a prepreg composed of resin layers having a different reaction rate. This method provides only insufficient improvement in the dimensional stability when applied to the fabrication of laminated boards. In addition, the method may cause migration of voids in the inner cloth, resulting in impaired long-term reliability.
A method of improving the problem of resin powder production from prepregs when bending the substrates has been disclosed in Japanese Patent Publication 334/1994. The Japanese Patent Publication 334/1994 proposes a method of melting the parts from which resin powder may readily fall out or the parts to which the resin powder has become attached. Although this method can prevent production of resin powder from prepregs, the method has problems such as denaturing of epoxy resin due to melting, requirement of investment for equipment, and an increase in the number of process steps.
Therefore, an object of the present invention is to provide a process for manufacturing a prepreg having high thickness precision, free from production of resin powder by bending and the like, free from voids in the inner layers, free from flow out, and exhibiting excellent formability.
In view of this situation, the inventors of the present invention have conducted extensive studies and have found that a prepreg having high thickness precision, free from production of resin powder by bending and the like, free from voids in the inner layers, free from flow out, and exhibiting excellent formability can be obtained by a process comprising a step of impregnating a glass fiber substrate with a solvent, a step of impregnating the solvent-impregnated glass fiber substrate with epoxy resin, a step of heating the epoxy resin-impregnated glass fiber substrate under specific conditions, a step of further impregnating the epoxy resin-impregnated glass fiber substrate, in which the epoxy resin has been cured, with the epoxy resin, and a step of heating the last-obtained epoxy resin-impregnated glass fiber substrate under specific conditions, wherein the epoxy resin reaction rate in the inner layer is 85% or more and the epoxy resin reaction rate in the outer layer is 60% or less. These findings have led to the completion of the present invention.
Specifically, the present invention provides a process of manufacturing a prepreg comprising (a) a step of impregnating a glass fiber substrate with a solvent, (b) a step of impregnating the solvent-impregnated glass fiber substrate with epoxy resin, (c) a step of heating the epoxy resin-impregnated glass fiber substrate, (d) a step of further impregnating the epoxy resin-impregnated glass fiber substrate, in which the epoxy resin has been cured, with the epoxy resin, and (e) a step of heating the lastly obtained epoxy resin-impregnated glass fiber substrate, wherein the epoxy resin reaction rate in the inner layer is 85% or more and the epoxy resin reaction rate in the outer layer is 60% or less.